Multipass Absorption Cell

Abstract

Laser absorption spectroscopy using multi-pass absorption based on multi-plane photoconversion is a highly sensitive chemical sensing technique for determining the molecular composition and concentration of samples, especially gas samples. To achieve long optical path lengths while keeping the detector small, many trace gas sensors rely on multi-pass absorption cells in which the beam is reflected multiple times. The novel approach of the present invention is to use a multi-plane photoconversion (MPLC) phase plate as a reflector in such a multi-pass absorption cell.

Description

[Technical field]

[0001] The present invention relates to multi-pass absorption cells based on multi-faceted light conversion for spectroscopy, and in particular to the design and fabrication of such cells. [Background technology]

[0002] Laser absorption spectroscopy is a highly sensitive and selective chemical sensing technique that determines the molecular composition and concentration of a sample by measuring its absorption spectrum with a laser source. Since the absorbance A of a sample is proportional to its concentration and the optical path length L, as described by the Beer-Lambert law, by increasing L it is possible to detect molecules at lower concentrations. This is particularly relevant for gases, which typically have lower absorption per unit length, i.e. lower extinction coefficients, than solid-state samples. To take advantage of long optical path lengths while maintaining a small form factor of the detector, many trace gas sensors rely on multi-pass absorption cells, where the beam is reflected multiple times to maximize the interaction length in a relatively small volume. The best known multipass cell designs are described by J.U. White in "Long Optical Paths of Large Aperture," J.Opt.Soc.Am.32(5), 285 (1942), and by D.Harriott, H.Kogelnik, and R.Kompfner in "Off-Axis Paths in Spherical Mirror Interferometers," Appl.Opt.3(4), 523 (1964). Both designs are based on spherical mirrors.

[0003] A variety of novel configurations have been introduced over the years to improve the path-to-volume ratio and performance. J.B. McManus et al., in "Astigmatic mirror multipass absorption cells for long-path-length spectroscopy", Appl. Opt. 34(18), 3336 (1995), introduced a variation of the Herriott design based on an astigmatic mirror, achieving a cell with a path length of 100 m for a volume of 3 liters and a cell with a path length of 36 m for a volume of 0.3 liters.

[0004] More recently, Krzempek et al. in "CW DFB RT Diode Laser-Based Sensor for Trace-Gas Detection of Ethane Using a Novel Compact Multipass Gas Absorption Cell", Applied Physics B 112, n°4 (September 2013), 461-65, demonstrated a high-density patterned multipass cell using two spherically aberrated mirrors. The optical path length in the cell was 57.6 m long with a volume of 270 cm3, or 0.27 liters.

[0005] In "Compact multipass optical cell for laser spectroscopy", Opt. Lett. 38(3), 257 (2013), B. Tuzson et al. proposed and demonstrated a more compact design based on a single reflecting toroidal surface that provides optical path lengths of up to 7.1 m in a volume of only 40 cm3 (0.04 liters).

[0006] Multipass cells are difficult to manufacture. In particular, as described by B. Tuzson et al. above, scattering at the mirror surface or any stray light can prevent the practical use of this type of cell for absorption spectroscopy by causing interference fringes. Existing solutions are based on large mirrors, which require significant volume and alignment, as described by Krzempek et al. above and references therein.

[0007] The present invention uses a novel approach to realize a compact, long-path multi-pass cell based on multi-plane optical conversion by using multi-plane optical conversion (MPLC).

[0008] Multi-plane optical transformation (MPLC) is a low-loss beam shaping process that makes it possible to perform any desired unitary transformation of optical modes, as described in "Programmable Unitary Spatial Mode Manipulation" by JF Morizur et al., Journal of the Optical Society of America A 27, no. 11 (November 1, 2010): 2524.

[0009] MPLC is ideal for systems with multiple beams because it can shape the beams simultaneously. MPLC can be implemented in both transmissive and reflective configurations. A particularly attractive implementation of MPLC for multi-pass absorption cells relies on multiple reflections between a reflective phase plate and a mirror, as disclosed in several patents and patent applications, e.g., US 10382133(B2), WO 2018134534(A1), WO 2016037850(A1), and in the paper by Guillaume Labroille et al. entitled "Characterization and Applications of Spatial Mode Multiplexers Based on Multi-Plane Light Conversion", Optical Fiber Technology 35 (February 2017):93-99. The paper discloses that MPLC can perform arbitrary unitary transformations. This is exemplified by demonstrating the injection of light from ten separate single-mode fibers into orthogonal modes of a multimode fiber, as shown in Figure 2 of the paper. MPLC is realized by injecting light from a linear array of single-mode fibers onto a phase plate and exiting into a multimode fiber after 14 bounces between the phase plate and a mirror. The phase plate geometry defines the light transformation and is optimized to achieve mode conversion from the 10 fundamental modes of the single-mode fiber to one of the modes of the multimode fiber. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] US Patent No. 10382133 (B2) [Patent Document 2] International Publication No. 2018134534(A1) Brochure [Patent Document 3] International Publication No. 2016037850(A1) Brochure [Non-patent literature]

[0011] [Non-Patent Document 1] JUWhite, “Long Optical Paths of Large Aperture”, J.Opt.Soc.Am.32(5), 285(1942) [Non-Patent Document 2] D. Harriott, H. Kogelnik and R. Kompfner, "Off-Axis Paths in Spherical Mirror Interferometers", Appl. Opt. 3(4), 523 (1964) [Non-Patent Document 3] JBMcManus et al., "Astigmatic mirror multipass absorption cells for long-path-length spectroscopy", Appl.Opt.34(18), 3336(1995) [Non-Patent Document 4] Krzempek et al., “CW DFB RT Diode Laser-Based Sensor for Trace-Gas Detection of Ethane Using a Novel Compact Multipass Gas Absorption Cell”, Applied Physics B 112, n°4 (September 2013): 461-65 [Non-Patent Document 5] B.Tuzson et al., “Compact multipass optical cell for laser spectroscopy”, Opt.Lett.38(3), 257(2013) [Non-Patent Document 6] JFMorizur et al., “Programmable Unitary Spatial Mode Manipulation”, Journal of the Optical Society of America A 27, no. 11 (1 November 2010): 2524 [Non-Patent Document 7] Guillaume Labroille et al., “Characterization and Applications of Spatial Mode Multiplexers Based on Multi-Plane Light Conversion”, Optical Fiber Technology 35(February 2017):93-99 Summary of the Invention [Problem to be solved by the invention]

[0012] These approaches result in small volumes that are advantageous for many applications. Furthermore, reflective MPLC systems avoid the dispersion and absorption that result from propagation through refractive optics. The present invention improves upon this technology to realize small-volume, long-path gas cells.

[0013] US Patent No. 10382133 (B2) describes MPLC as an optical phase-shifting component used to shift the phase and modify the intensity of a light beam injected into a fiber. The component is inserted somewhere in the fiber. It uses two mirrors and multiple beam paths between the mirrors. An optical phase-shifting structure, for example a reflective phase mask with a structured surface, and finally a mirror, is effective at each reflection of the beam, gradually splitting it into fast and slow propagating modes. The fast mode undergoes one or more reflections more than the slow mode, thereby slowing it down. The fast and slow modes are recombined and transmitted in a multimode fiber where the modes have different propagation speeds. However, this design must be modified to be able to be used in gas cells. [Means for solving the problem]

[0014] The MPLC system allows to modify the optical modes of 50 bundled single mode fibers and inject them into 50 higher mode multimode fibers with very little crosstalk and low losses. Assuming that the modes are characterized by their direction, shape, phase front and position, and that in the MPLC any mode can be designed to be arbitrarily converted into any other mode, it is possible to repurpose the MPLC with the following characteristics: Instead of placing several single mode fibers at the input, two fibers and a mirror perpendicular to the fiber optical axis are placed. At the exit, a mirror perpendicular to the exit optical axis is placed. Instead of modifying the mode shape, the position and direction of the mode at the exit reflecting surface is modified so that for each exit beam, there is an accepted reflected beam that travels in the opposite direction towards a mirror placed next to the input fiber. The same design is applied to the mirror placed next to the input fiber, and the beam that leaves the input side towards the mirror is reflected and injected into the accepted modes of the MPLC and returns to the original exit side. This process continues until the last trip through the MPLC is performed, at which point the MPLC is designed to inject light into the exit fiber.

[0015] Considering a centimeter-sized MPLC with 10 bounces and a 50 bounce path, we obtain a multipath cell with an effective length of 5 meters, comparable to current commercial cells such as those described by J.B. McManus et al. and B. Tuzson et al. Furthermore, whereas existing commercial cells are machined, MPLCs can be manufactured using planar semiconductor manufacturing processes, which have the advantage of parallelization, i.e., manufacturing many units simultaneously on a single wafer. Thus, the volumetric price of the described novel embodiment can be significantly lower than machined versions.

[0016] The accompanying drawings show: [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram of a multi-pass gas cell based on MPLC. [Figure 2a] FIG. 1 is a schematic top view of a free-space coupled multi-pass gas cell based on MPLC. [Figure 2b] FIG. 2b is an isometric view of the cell of FIG. 2a. [Figure 3a] FIG. 1 is a schematic top view of a fiber-coupled multi-pass gas cell based on MPLC. [Figure 3b] FIG. 3b is an isometric view of the cell of FIG. 3a. [Figure 4a] This is an example of a three-row square reflection pattern on a cell mirror. [Figure 4b] Here is an example of a similar 3 row hexagon pattern. [Figure 5a] FIG. 1 is a schematic top view of a fiber-coupled multi-pass gas cell based on MPLC. [Figure 5b] FIG. 5b is an isometric view of the cell of FIG. 5a. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Specific embodiments of the present invention are described below, with the understanding that the embodiments described herein are merely examples, and that those skilled in the art may utilize other embodiments without departing from the scope of the present invention, as defined in the claims.

[0019] Based on the above mentioned Labroille paper, it can be seen that using the MPLC in reverse, i.e. injecting at the output, provides an inverse unitary transformation, thus injecting many modes from a multimode fiber and converting it back into 10 separate single modes. A possible embodiment of a spectroscopic cell can be provided by placing a second MPLC before the first, so that the length of the multimode fiber is reduced to 0. The output of this second MPLC is again 10 single mode fibers, the first MPLC can be connected to the second MPLC, at the input the second MPLC can be connected to a third MPLC, at the output the third MPLC can be connected to a fourth MPLC, and so on. The light in this configuration travels 10 times through the device performing 14 reflections at each MPLC.

[0020] This embodiment is not optimal because it uses two MPLCs, each of which must be injected into the fiber and back into the MPLS on the entire trip through the system. A first improvement is to use a single phase plate that employs the same phase plate design on a plate twice as large, using 28 bounces by replicating the original pattern in the first half and a mirrored version of it in the second half. An additional improvement is then to change the orientation of the exit modes so that they face each other in pairs, and are reinjected into the MPLC by a plane mirror placed at the fiber array position where only the first and last two fibers are retained at the input, and at the opposite exit the plane mirror covers all fiber positions.

[0021] In the above embodiment, we convert the injected single mode into the mode of the multimode fiber and back into single mode. This is a wasteful conversion and is probably suboptimal. To achieve a multipass cell suitable for spectroscopic applications, we would like to optimize the system differently.

[0022] Three features or characteristics appear to result in an optimized embodiment.

[0023] First, because light bounces between the phase plate and the mirror multiple times, the phase plate and the mirror should provide the highest reflectivity at the operating wavelength. The higher the reflectivity, the less reflection losses accumulate.

[0024] Second, independent of the first, only the injection and exit modes coming from the laser are boundary conditions for optimization. The injection modes can have slight variations from manufacturing variations, which can affect the energy distribution and shape at long distances. The exit modes must be easy to collect by the detector, and in fact small enough to contain the detector and to be able to manufacture it. This condition maximizes the signal-to-noise ratio and minimizes nonlinearities.

[0025] Third, the intermediate unitary transformation resulting in the intermediate mode has no additional constraints on the geometry other than allowing the mode of the laser to be launched to the detector, but for applications it is necessary to realize functionality that can still provide functionality if the input wavelength is changed, typically by one wavenumber, or if the temperature is changed, or if the physical alignment of the laser with respect to the system is changed.

[0026] To optimize the second and third attributes, a figure of merit can be defined that can be used as an optimization function for an automated design search, such as a genetic algorithm or similar technique. The goal is to be able to scan a small wavelength range to observe the changes in absorption of the gas present between the mirrors of the MPLC and to avoid that these changes in attenuation are masked by irrelevant fluctuations. Such fluctuations can occur when the total transmission rate of the system changes because of wavelength changes or because there is crosstalk between modes that have bounced different times. In addition, acoustic vibrations can cause interference or loss of signals. To evaluate the signal-to-noise ratio, the output signal can be simulated in a given modality for the alignment of the laser, the mirrors facing each other, the wavelength, and the temperature. The output and its variance can then be averaged. The modality for the alignment of the laser and the mirrors must be chosen to reflect the range of errors introduced by the acoustic pressure. Furthermore, the modality for the temperature range must reflect the range of precision with which a temperature controller of reasonable cost can maintain the MPLC and the flowing gas, and the modality for the wavelength must explore the full wavelength range required to complete the spectroscopy. A sufficiently large number of different modality implementations must be explored to ensure that there is no aliasing due to undersampling of the modalities. The criterion is that doubling the number of samples does not change the signal-to-noise ratio by as much as the difference between the signal-to-noise ratios of alternative configurations of the same number of modalities.

[0027] This optimization maximizes performance, which may come at the cost of very difficult alignment, temperature stability or acoustic quietness. To optimize the manufacturing parameters, calculate the achievable mechanical precision and temperature stability at a given cost. The pool of solutions from the first step that reached a sufficiently high signal-to-noise ratio is re-evaluated, this time sampling for the modalities of the manufacturing setup, using the portion that reached acceptable performance as a figure of merit for the final selection of candidate designs.

[0028] The overall structure of the MPLC-based multi-pass gas cell according to the present invention is shown in Figure 1. The cell is composed of a reflective MPLC phase plate, a mirror, and an airtight enclosure with gas inlets and outlets and optical windows. The purpose of the enclosure is to keep the sample gas under characterization separate from the surrounding atmosphere and to control the gas pressure inside the cell. The airtight enclosure must include at least one optical window to allow the input beam to enter the cell and the output beam to exit the cell.

[0029] A first embodiment of a multi-pass gas cell according to the invention is a single beam cell as shown in Figures 2a and 2b. No airtight enclosure is shown. In this embodiment, the input laser beam propagates in free space. The beam undergoes multiple reflections between the phase plate and the mirrors, creating a long optical path length in a small volume. The phase plate is designed to compensate for the inherent divergence of the beam due to diffraction to maintain a constant diameter reflection after reflection. The phase plate can be designed to collimate the output beam if it needs to propagate over a long distance, or to focus the output beam a short distance from the cell, where the detector is located.

[0030] The optical path length L in the cell is

number

number

[0031] For example, in a cell with N = 20 reflections at the mirror with a distance d = 10 cm between the mirror and the phase plate, the optical path length is approximately equal to L = 4.2 m.

[0032] The phase plate is designed in this first embodiment to match the laser beam propagating in free space. To achieve this, a reflective phase plate is used. The desired spatial phase profile Φ(x,y) is realized by etching a corresponding height profile z(x,y) in a substrate, for example a glass plate. A highly reflective coating, typically a metal film, is then deposited on the surface to maximize its reflectivity.

[0033] To reproduce the beam diameter and beam divergence of the input beam at the output of the cell, in this first embodiment, the N phase profiles corresponding to the N reflections at the phase plate are N identical concave spherical or parabolic surfaces. This arrangement produces a periodic sequence of free space propagations over a distance 2d, followed by a focusing mirror of focal length f. For such a periodic system to support stable ray trajectories that are repeated after N periods, the focal length must satisfy the condition

number

[0034] For a spherical mirror, the focal length A of the mirror is related to its radius of curvature R by R = 2f. The radius of curvature is therefore determined by the condition

number

[0035] A phase plate consisting of N spherical mirrors satisfying this condition will reproduce the diameter and divergence of the input beam at the output of the cell.

[0036] As shown generally in Figures 3a and 3b, in a second embodiment of the present invention, a single beam is delivered to a multipass cell by an optical fiber and collimated by a collimator lens attached to the end of a fiber patch cable.

[0037] Due to the great flexibility in designing and fabricating MPLC phase plates, dense multi-row reflection patterns can be generated on the mirror. By stacking M rows of multiple reflections in the vertical direction, as shown in Figures 4a and 4b for M=3,

number

[0038] For example, for a cell with M = 3 rows and N = 20 reflections per row at the mirror with a distance d = 10 cm between the mirror and the phase plate, the optical path length is approximately equal to L = 12.6 m.

[0039] Due to the flexibility in design and manufacturing of MPLC phase plates, it is possible to generate multi-row reflection patterns on the mirrors arranged in a hexagonal lattice (also called a triangular lattice), which is the densest tiling of circles in two dimensions.

[0040] If an application requires detecting multiple gases with non-overlapping absorption spectra, the tuning range of a single laser is not sufficient and several lasers need to be integrated into a single sensor cell.

[0041] Multipass cells require very precise control of the input beam position and angle of incidence to achieve the specified performance. Therefore, combining multiple laser beams into a single multipass cell is difficult. The beams need to be made parallel and collinear using external beam combining optics before entering the cell. This is typically achieved using polarized beam combining, and / or spectral beam combining with dichroic mirrors or diffraction gratings.

[0042] Multi-faceted optical conversion has been shown to be a powerful technique for beam combining, see for example the two references by G. Labroille et al. mentioned above. The output beams of several single-mode optical fibers can be efficiently multiplexed into a single multimode fiber by converting them into orthogonal spatial modes.

[0043] The phase plate for this second embodiment is similar to the one defined in relation to the first embodiment.

[0044] In another embodiment of the invention, multiple non-collinear beams propagate together into an MPLC multipass cell and are combined at the output of the cell towards a detector. In this multibeam cell configuration, shown in Figures 5a and 5b, the MPLC phase plates and mirrors serve two different functions: a) to create a long optical path length in a small volume, and b) to combine multiple input beams. In contrast to conventional multipass cells, there is no need to combine the beams before entering the cell. The beams are delivered to the cell by a fiber bundle and collimated by a microlens array, as shown in Figure 5a.

[0045] To accept M parallel, non-collinear beams at the input and direct them to a single detector at the output, the MPLC phase plate of this embodiment can be constructed, for example, as follows: Each individual phase profile, except for the last, consists of M stacked spheres or paraboloids, with a radius of curvature determined for the single beam cell. The last phase profile consists of M stacked planes, angled to direct the M individual beams to intersect at a single point where the detectors are located. This is only one example, and there are other combinations of phase profiles that can be used to realize such a cell.

[0046] For gas sensors utilizing semiconductor lasers, such as near-infrared diode lasers or mid-infrared quantum cascade lasers, it is possible to reduce the size of the entire sensor, including the light source, gas absorption cell, and photodetector as well as the multipass cell, by connecting or integrating the laser chip and its collimation optics to the entrance of the cell instead of an optical fiber and its collimation optics.

[0047] The same is true for multi-gas sensors, which require multiple lasers. Instead of a fiber optic bundle delivering light from several laser sources, a semiconductor laser array chip containing multiple emitters with different wavelengths can be attached or integrated at the entrance of the cell and collimated by a microlens array.

[0048] In summary, a novel multipass cell architecture based on reflective MPLC phase plates and mirrors is presented. This architecture is compatible with free-space and fiber coupling of laser sources. It allows to create long optical path lengths in a small volume by generating very dense reflection patterns, including hexagonal lattice patterns, which are the densest arrangement of circles.

[0049] The disclosed MPLC multipass cell architecture offers the unique possibility of combining multiple spatially separated laser beams without prior beam combining. Beam combining is achieved by the cell itself, and the beams are sent to a single detector at the exit. This multibeam configuration can be realized with a fiber bundle to deliver the beams to the cell and a microlens array for collimation, or in the case of semiconductor lasers, with a laser array chip mounted at the entrance of the cell and a microlens array for collimation.

[0050] MPLCs can be manufactured using planar manufacturing processes, which have the advantage of parallelization, i.e., manufacturing many units simultaneously on a single wafer. Thus, the volumetric price for manufacturing multipass cells based on MPLCs can be significantly lower than current machined cells.

Claims

1. A multi-pass gas absorption cell comprising at least two reflecting means arranged to increase the total optical path length of a laser beam within an enclosure or chamber containing the gas to be analyzed, characterized in that at least one of the reflecting means is a reflective multi-plane photoconversion (MPLC) phase plate.

2. 2. The gas absorption cell of claim 1, wherein to analyze the gas at multiple wavelengths, several spatially distinct laser beams of different wavelengths entering the chamber are converted into orthogonal spatial modes that are reflected by the reflective MPLC phase plate in the chamber and measured by a single detector.

3. 10. The gas absorption cell of claim 1, wherein the chamber includes at least one optical mirror, an inlet and an outlet for the gas, and at least one optical window for a laser beam.

4. 4. A gas absorption cell according to claim 1, wherein the MPLC phase plate exhibits a spatial phase profile that matches a laser propagating in free space, the spatial phase profile being achieved by generating a corresponding height profile of a reflective surface of the MPLC phase plate.

5. 4. A gas absorption cell according to claim 1, wherein the MPLC phase plate exhibits a spatial phase profile that matches a laser propagating through an optical fiber, the spatial phase profile being achieved by generating a corresponding height profile of a reflective surface of the MPLC phase plate.

6. 6. A gas absorption cell according to claim 4 or 5, wherein the spatial phase profile is generated by etching a desired height profile into the surface of the MPLC phase plate, preferably by fabricating several MPLC phase plates on a single glass wafer using planar manufacturing techniques, and by depositing a metal film on the surface to enhance reflectivity.

7. 7. A gas absorption cell according to claim 1, wherein N phase profiles corresponding to N reflections on the MPLC phase plate are provided by N identical concave spherical or parabolic mirrors on the surface.

8. 8. The gas absorption cell of claim 7, wherein the N mirrors are arranged in a stacked array of mirrors to produce an increased optical path length.

9. 5. A gas absorption cell according to claim 3 or 4, wherein a single beam is delivered to the cell via an optical fibre, the fibre having a collimator at the end facing the cell.

10. 10. The gas absorption cell according to any one of claims 1 to 9, wherein the multi-plane light conversion (MPLC) phase plate is a glass structure made by a planar manufacturing process, preferably by making several phase plates on a single wafer.

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